Field of the Invention
This invention relates generally to the field of wireless communication; and more particularly, to an active antenna system adapted to utilize a shared tunable component for both active band switching and impedance matching for reduced component volume and efficient antenna operation.
Description of the Related Art
Current and future communication systems will require antenna systems capable of operation over multiple frequency bands. Efficiency improvements in the antenna system will be needed to provide better overall communication system performance, for example, increased antenna efficiency will translate into greater battery life in a mobile wireless device. For Multiple Input Multiple Output (MIMO) applications isolation between multiple antennas as well as de-correlated radiation patterns will need to be maintained across multiple frequency bands. Closed loop active impedance matching circuits integrated into the antenna will provide for the capability to dynamically impedance match the antenna for a wide variety of use conditions, such as the handset against the user's head for example. These and other requirements continue to drive a need for dynamic tuning solutions, such as active frequency shifting, active beam steering, and active impedance matching, such that antenna characteristics may be dynamically altered for improving antenna performance.
Commonly owned U.S. Pat. No. 7,911,402, issued Mar. 22, 2011, and titled “ANTENNA AND METHOD FOR STEERING ANTENNA BEAM DIRECTION”, describes a beam steering technique wherein a single antenna is capable of generating multiple radiating modes; the contents of which are hereby incorporated by reference. In sum, this beam steering technique is effectuated with the use of a driven antenna and one or more offset parasitic elements that alter the current distribution on the driven antenna as the reactive load on the parasitic is varied. Multiple modes are generated, and thus this technique can be referred to as a “modal antenna technique”, and an antenna configured to alter radiating modes in this fashion can be referred to as an “active multimode antenna” or “active modal antenna”.
FIGS. 1(a-c) illustrate an example of an active modal antenna in accordance with the '402 patent, wherein FIG. 1a depicts a circuit board 11 and a driven antenna element 10 disposed thereon, a volume between the circuit board and the driven antenna element forms an antenna volume. A first parasitic element 12 is positioned at least partially within the antenna volume, and further comprises a first active tuning element 14 coupled therewith. The first active tuning element 14 can be a passive or active component or series of components, and is adapted to alter a reactance on the first parasitic element 12 either by way of a variable reactance, or shorting to ground, resulting in a frequency shift of the antenna. A second parasitic element 13 is disposed about the circuit board and positioned outside of the antenna volume. The second parasitic element 13 further comprises a second active tuning element 15 which individually comprises one or more active and passive components. The second parasitic element 13 is positioned adjacent to the driven element 10 and yet outside of the antenna volume, resulting in an ability to shift the radiation pattern characteristics of the driven antenna element by varying a reactance thereon. This shifting of the antenna radiation pattern can be referred to as “beam steering”. In instances where the antenna radiation pattern comprises a null, a similar operation can be referred to as “null steering” since the null can be shifted to an alternative position about the antenna. In the illustrated example, the second active tuning element 15 comprises a switch for shorting the second parasitic to ground when “On” and for terminating the short when “Off”. It should however be noted that a variable reactance on either of the first or second parasitic elements, for example by using a variable capacitor or other tunable component, may further provide a variable shifting of the antenna pattern or the frequency response. FIG. 1b illustrates a two-dimensional antenna radiation pattern associated with the modal antenna of FIG. 1a, wherein the pattern is shifted upon actively configuring the second parasitic element 13 from a first mode 16 to a second mode 17, or a third mode 18. FIG. 1c illustrates a typical frequency plot of the modal antenna of FIG. 1a, the frequency of the antenna can be shifted by actively configuring the first parasitic element 12 of the modal antenna. Here, a first frequency (f0) of the antenna is achieved when the first and second parasitic elements are switched “Off”; the frequencies (fL) and (fH) are produced when the second parasitic is shorted to ground; and the frequencies (f4; f0) are produced when the first and second parasitic elements are each shorted to ground. Further details of this type of modal antenna can be understood upon a review of the '402 patent.
An early application identified for use with such active modal antennas includes a receive diversity application described in commonly owned U.S. patent application Ser. No. 13/674,137, filed Nov. 12, 2012, and titled “MODAL ANTENNA WITH CORRELATION MANAGEMENT FOR DIVERSITY APPLICATIONS”, wherein a single modal antenna can be configured to generate multiple radiating modes to provide a form of switched diversity; the contents of which are hereby incorporated by reference. Certain benefits of this technique include a reduced volume required within the mobile device for a single antenna structure instead of a the volume required by a traditional two-antenna receive diversity scheme, a reduction in receive ports on the transceiver from two to one, and the resultant reduction in current consumption from this reduction in receive ports and associated conductive surfaces.
With Multiple Input Multiple Output (MIMO) systems becoming increasingly prevalent in the access point and cellular communication fields, the need for two or more antennas collocated in a mobile device or small form factor access point are becoming more common. These groups of antennas in a MIMO system need to have high, and preferably, equal efficiencies along with good isolation and low correlation. For handheld mobile devices the problem is exacerbated by antenna detuning caused by the multiple use cases of a device: hand loading of the cell phone, cell phone placed to user's head, cell phone placed on metal surface, etc. For both cell phone and access point applications, the multipath environment is constantly changing, which impacts throughput performance of the communication link.
Commonly owned U.S. patent application Ser. No. 13/289,901, filed Nov. 4, 2011, and titled “ANTENNA WITH ACTIVE ELEMENTS”, describes an active antenna wherein one or multiple parasitic elements are positioned within the volume of the driven antenna. The impedance at the junction of the parasitic element and the ground plane is altered to effectuate a change in the resonant frequency of the antenna. For a driven antenna that is designed to contain multiple resonances at several frequencies, the multiple resonances can be shifted in frequency utilizing one or multiple parasitic elements. This results in a dynamically tunable antenna structure where the frequency response can be altered to optimize the antenna for transmission and reception over a wider frequency range than could be serviced by a passive antenna.
These and other active modal antenna techniques drive a need for a module or other circuit having active components for coupling with or integrated into the antenna. Such active components may include tunable capacitors, tunable inductors, switches, PIN diodes, varactor diodes, MEMS switches and tunable components, and phase shifters. Additionally, passive components may further be incorporated into such modules and other circuits for driving active antennas, whereas the passive components may include capacitors, inductors, and transmission lines with fixed and variable electrical delay for tuning the antenna. Additionally, there is a present and ongoing need for such active antennas adapted for band switching and impedance matching using fewer components such that costs and circuit volume can be reduced.